Flexible and conformable stent and method of forming same

ABSTRACT

An expandable stent is fabricated from a plurality of circumferential rings, each pair of adjacent circumferential rings being interconnected by a multiplicity of links, each link contacting a portion of a strut in one ring along one longitudinal axis and a portion a strut in a directly adjacent ring along a different longitudinal axis. Each link is circumferentially S-shaped and adjacent links are mirror images of each other. The resulting stent is both flexible and strong when expanded thereby providing good scaffolding. In one embodiment, the strut material is fabricated of shape memory alloy such that when the strut is compressed so as to be insertable into a vessel, the strut will automatically expand to its previous dimension upon rising to the temperature of the vessel in which it is placed, thereby to expand and hold open the vessel without need for a balloon catheter.

FIELD OF THE INVENTION

The present invention relates to an expandable stent.

BACKGROUND OF THE INVENTION Description of the Prior Art

Stents are generally known. Indeed, the term “stent” has been usedinterchangeably with terms such as “intraluminal vascular graft” and“expansible prosthesis”. As used throughout this specification the term“stent” is intended to have a broad meaning and encompasses anyexpandable prosthetic device for implantation in a body passageway(e.g., a lumen or artery).

In the past fifteen years, the use of stents has attracted an increasingamount of attention due the potential of these devices to be used, incertain cases, as an alternative to surgery. Generally, a stent is usedto obtain and maintain the patency of the body passageway whilemaintaining the integrity of the passageway. As used in thisspecification, the term “body passageway” is intended to have a broadmeaning and encompasses any duct (e.g., natural or iatrogenic) withinthe human body and can include a member selected from the groupcomprising: blood vessels, respiratory ducts, gastrointestinal ducts andthe like.

Stent development has evolved to the point where the vast majority ofcurrently available stents rely on controlled plastic deformation of theentire structure of the stent at the target body passageway so that onlysufficient force to maintain the patency of the body passageway isapplied during expansion of the stent.

Generally, in many of these systems, a stent, in association with aballoon, is delivered to the target area of the body passageway by acatheter system. Once the stent has been properly located (for example,for intravascular implantation, the target area of the vessel can befilled with a contrast medium to facilitate visualization duringfluoroscopy), the balloon is expanded thereby plastically deforming theentire structure of the stent so that the latter is urged in placeagainst the body passageway. As indicated above, the amount of forceapplied is at least that necessary to expand the stent (i.e., theapplied force exceeds the minimum force above which the stent materialwill undergo plastic deformation) while maintaining the patency of thebody passageway. At this point, the balloon is deflated and withdrawninto the guide catheter, and is subsequently removed. Ideally, the stentwill remain in place and maintain the target area of the body passagewaysubstantially free of blockage (or narrowing).

See, for example, any of the following patents:

-   -   U.S. Pat. No. 4,733,665 (Palmaz),    -   U.S. Pat. No. 4,739,762 (Palmaz),    -   U.S. Pat. No. 4,800,882 (Gianturco),    -   U.S. Pat. No. 4,907,336 (Gianturco),    -   U.S. Pat. No. 5,035,706 (Gianturco et al.),    -   U.S. Pat. No. 5,037,392 (Hillstead),    -   U.S. Pat. No. 5,041,126 (Gianturco),    -   U.S. Pat. No. 5,102,417 (Palmaz),    -   U.S. Pat. No. 5,147,385 (Beck et al.),    -   U.S. Pat. No. 5,282,824 (Gianturco),    -   U.S. Pat. No. 5,316,023 (Palmaz et al.),    -   U.S. Pat. No. 5,755,771 (Penn et al.),    -   U.S. Pat. No. 6,183,506 (Penn et al.), and    -   U.S. Pat. No. 6,217,608 (Penn et al.),        for a discussion of previous stent designs and deployment        systems. See also the book entitled “Handbook of Coronary        Stents” Fourth Edition, edited by Serruys and Rensing, copyright        2002, Martin Dunitz Ltd., for a description on pages 109 to 116        of the prior art Genius Coronary Stent using two S shaped        circumferentially oriented links to connect two adjacent        expandable undulating rings.

Two of the functional constraints which govern the usefulness of a stentare first, the stent should have a high degree of flexibility in theunexpanded state to facilitate navigation of the stent through tortuousanatomy to the location of the target stenosis and second, the expandedstent should be radially rigid to minimize the effects of restenosis andthe possibility of acute occlusion. Thus, an ideal stent would becharacterized by certain functional properties (i.e. flexibility anddimensional stability) independent of the state of the stent (i.e.,expanded or unexpanded).

Prior art stents often achieve flexibility at the expense of the abilityto support radially the vessel wall and to deliver medicines or drugsuniformly to the vessel wall. Moreover, such stents, when expanded,often lose flexibility and conformability.

Accordingly, it would be desirable to have an improved stent whichovercomes these disadvantages (i.e. has improved radial support,improved uniformity of drug delivery, improved flexibility duringdelivery, and improved conformability) can be manufactured readily, andcan be deployed using conventional stent delivery systems.

SUMMARY OF THE INVENTION

The present invention obviates or mitigates at least some of theabove-mentioned disadvantages of the prior art.

Accordingly, in one of its aspects, the present invention provides anunexpanded stent comprising:

-   -   a plurality of radially expandable undulating rings, each        radially expandable undulating ring comprising a strut with a        plurality of apices;    -   a plurality of arcuate flex members connecting adjacent first        and second radially expandable undulating rings;

the first radially expandable undulating ring comprises a strut withmore than one complete sinusoidal cycle which is coupled to twocircumferentially adjacent arcuate flex members; and

-   -   the second radially expandable undulating ring comprises a strut        with less than one complete sinusoidal cycle which is coupled to        two circumferentially adjacent arcuate flex members.

In another of its aspects, the present invention provides a stentcomprising:

-   -   a first radially expandable undulating ring comprising a first        apex;    -   a second radially expandable undulating ring comprising a second        apex longitudinally unaligned with respect to the first apex;        and    -   a mirrored pair of arcuate flex members interconnecting the        first ring and the second ring.

The novel stent design of this invention has several unique features. Inone embodiment, more than two cycles of the radially expandableundulating struts are located adjacent one pair of arcuate flex memberswhich connect adjacent ones of the first and second radially expandableundulating rings. The links between a pair of radially expandableundulating struts are each the mirror image of the other. While at leastone prior art stent uses a longitudinal sinusoidal link, in thisembodiment the invented stent utilizes circumferentially S-shaped linkswith adjacent links being mirror images of each other.

One advantage of the invented stent is that over a given length of thestent there are more expansion rings and more links so the stent hasgreater flexibility, more even distribution of support, more uniformdrug distribution and better conformability than the prior art. Whilethe ideal stent has infinite flexibility, the invented stent has muchgreater flexibility than the prior art. The invented stent also hasexcellent scaffolding properties; i.e., the invented stent supports theblood vessels and other lumens by having a relatively uniformdistribution of support structure in the expanded state. This isachieved by causing the stent to maintain a more constant distributionof the struts over the length of the stent compared to prior art stentswhich helps structurally support the lumen thereby to keep open the flowpassage in which the stent is inserted. This feature also allows moreuniform drug delivery utilizing drugs coated onto or formed with thestent material.

The bending mechanism of the invented stent differs from the bendingmechanism of most prior art stents by using a circumferential ratherthan a longitudinal S-shape to link adjacent expansion struts. While inthe prior art longitudinal S-shapes link or connect the expansion strutrings, in one embodiment the invented stent structure usescircumferentially “S” shaped arcuate flex members with adjacent flexmembers being mirror images of each other, to connect pairs of radiallyexpanding undulating rings to give better pivoting. Because eachcircumferential “S” shaped link expands as the stent expands, stentflexibility is maintained as the stent expands.

The shape of the flex members interconnecting adjacent radiallyexpanding undulating rings helps maintain a continuous curve duringflexing of the stent while the stent is being inserted into a vessel.This greatly improves the insertability of the stent, reducing the riskof injury to the vessel. After expansion, this feature also allows thestent in its expanded state to follow the curvature of the vessel,thereby further preventing injury to the vessel. The invented structurewherein the longitudinal length of each expandable ring or strut isshort (typically less than two unexpanded stent diameters), hasdimensions such as to approximate a smooth curve when the stentstructure is bent.

As a result, the invented stent is quite flexible (i.e., trackable)which means the invented stent has the ability to go around cornersbetter than prior art stents. After the invented stent is expanded, thestent is longitudinally conformable to the lumen or vessel. This isimportant because some prior art stents while flexible when unexpanded,straighten out and stiffen up when expanded thereby attempting tostraighten out the vessel in which they are placed. The circumferentialS-links and the close spacing of the expansion rings both contribute tothe improved flexibility and more uniform vessel support of the inventedstent. This invented stent therefore combines good vessel wall support(i.e. “good scaffolding”) with good flexibility.

While the prior art categorizes the stents as “closed cell” (every apexin the cell boundary is tied to another apex) or “open cell” (one ormore apices in the cell boundary are not tied to another apex), theinvented stent structure has advantages of both “closed” and “open”cells without their significant disadvantages.

The closed cell tends to straighten out when inserted and expanded andthis is not good. In the invented stent, the link and expansion ringshave the same longitudinal spatial frequency but each link contactsadjacent rings on circumferentially different longitudinal lines whilesome prior art links contact adjacent rings on the same longitudinallines. In addition, each link is not directly connected to adjacentlinks. In one embodiment each nine expansion ring is made up of ninesinusoidally undulating struts but only six links are provided toconnect adjacent expansion rings. This structure allows the inventedstent to achieve better flexibility and better conformability than priorart stents.

In addition, the invented stent has more structural features around thecircumference which enables each ring to be longitudinally shorter andthus gives better resolution. The longitudinally shorter stent expansionrings enable better flexibility and conformability to the vessel.

In one embodiment of the invented stent, six links are placed around thecircumference and six connections are thus formed between adjacentexpansion rings. Of course, other numbers of link members can also beused in the spirit of the invention.

The invented stent reduces tissue prolapse, provides more uniformsupport distribution (i.e., more uniform scaffolding and radialstrength), and provides more uniform drug distribution. While sometimesrestenosis occurs after a stent has been placed in a vessel, drugcoatings delay a stent restenosis. So the material in the invented stentnot only coats well with a drug but more uniformly distributes the drugthan in the prior art.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the present invention will be described with referenceto the accompanying drawings, wherein like reference numerals denotelike parts, and in which:

FIGS. 1 a through 1 h show the stent of this invention in variousdegrees of expansion after having been placed in a lumen or vessel.

FIGS. 2 a, 2 b, 2 c and 2 d show various techniques for changing thephase transformation properties of selected portions of the cylindricalstent.

FIGS. 3 a, 3 b, 3 c and 3 d illustrate isomers associated with the stentcell of this invention and three prior art stent cells, respectively,and show in inches the minimum distance of the point that is farthestfrom the stent boundary for the stent cell of this invention and forthree (3) prior art stent cells respectively.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Described herein is a novel stent design which has flexibilitycharacteristics superior to currently available stent structures.Further, the expanded stent exhibits an inherent tendency to maintainthe longitudinal shape of the lumen in which it is deployed. This is asignificant advantage of the present stent compared to many currentlyavailable stents. Specifically, upon expansion, many currently availablestents tend to stretch and straighten and thus deform the lumen alongits length from its natural orientation.

Upon expansion, the repeating pattern in the present stent becomesexpanded, ideally to a truss-like shape, which results in very desirableradial rigidity. Further, the present stent has a desirable strutdensity when expanded thereby resulting in excellent vessel coveragewhile allowing side branch access.

Other advantages of the present stent design will be readily apparent tothose of skill in the art.

The present stent may be constructed from any suitable startingmaterial. Preferably, the starting material is a thin-walled tube of ametal, alloy, plastic or polymer. Alternatively, it is possible toconstruct the present stent from a flat sheet which preferably is cut toform a rectangle with two parallel long sides, (as described below),rolled until the two long sides abut and then welded.

In one preferred embodiment, the starting material may be one which isplastically deformable—non-limiting examples of such a material includestainless steel, titanium, tantalum and the like. In another preferredembodiment, the starting material may be one which expands viatemperature-dependent memory (i.e., a material which when deformed willchange back to its memory state upon reaching a certain temperature);non-limiting examples of such a material include nickel-titanium alloys(called for short “nitinol”), shape memory polymers, and the like.Alternatively, a material such as an elastic polymer may be used.

With reference to the accompanying Figures, there is illustrated atwo-dimensional view of one embodiment of the present stent. Thus, theillustrated views are derived by unrolling into a flat plane a sideelevation of a tubular stent.

The illustrated stent may be produced by any of a number of knowntechniques. For example, it is preferred to produce the present stent bystarting with a hollow, unpatterned tube material (i.e., cylindricalwall with no porous surface) and then removing portions of the tubematerial to define a porous surface. While the precise nature of thisprocessing is not particularly restricted, it is preferred to use acomputer programmable, laser cutting system which operates by:

-   -   a. receiving the solid tube,    -   b. moving the solid tube longitudinally and rotationally under a        laser beam to selectively remove sections of the solid tube        thereby defining a porous surface; and    -   c. cutting stent sections of desirable length of the solid tube.

A suitable such laser cutting system is known in the art as the LPLS-100Series Stent Cutting Machine. The operation of this system to producethe unexpanded stent is within the purview of a person skilled in theart.

Thus, the stent produced from the laser cutting system may be anunexpanded,expanded or intermediate state.

If desired, the stent material may have a coating material appliedthereon. The coating material may be disposed continuously ordiscontinuously on the surface of the stent. Further, the coating may bedisposed on the interior and/or the exterior surface(s) of the stent.The coating material may be one or more of a biologically inert material(e.g., to reduce the thrombogenicity of the stent) or a medicinalcomposition which leaches into the wall of the body passageway afterimplantation (e.g., to provide anticoagulant action, to deliver apharmaceutical to the body passageway and the like).

The stent is preferably provided with a biocompatible coating, in orderto minimize adverse interaction with the walls of the body vessel and/orwith the liquid, usually blood, flowing through the vessel. The coatingis preferably a polymeric material, which is generally provided byapplying to the stent a solution or dispersion of preformed polymer in asolvent and removing the solvent. Non-polymeric coating material mayalternatively be used. Suitable coating materials, for instancepolymers, may be polytetraflouroethylene or silicone rubbers, orpolyurethanes which are known to be biocompatible. Preferably, however,the polymer has zwitterionic pendant groups, generally ammoniumphosphate ester groups, for instance phosphoryl choline groups oranalogues thereof. Examples of suitable polymers are described inpublished International patent applications WO-A-93/16479 andWO-A-93/15775. Polymers described in those specifications arehemo-compatible as well as generally biocompatible and, in addition, arelubricious. When a biocompatible coating is used, it is important toensure that the surfaces of the stent are completely coated in order tominimize unfavourable interactions, for instance with blood, which mightlead to thrombosis. Also, bio-absorbable materials such as highmolecular weight poly-L-lactic acid can be used as part or all of thestent material.

This good coating can be achieved by suitable selection of coatingconditions, such as coating solution viscosity, coating technique and/orsolvent removal step. The coating, if present, can be applied to thestent in an expanded or contracted state.

FIG. 1 a illustrates in a plane the shape of the material which forms astent in accordance with this invention. In the actual stent, thepattern shown in FIG. 1 a is in a tubular form with the two edges of thematerial abutting. If the material in the tube was originally formed ina plane, then the material, after patterning, is rolled into a tubularform and the two abutting edges are welded to form the tube.Alternatively, as described above, the tube can be formed by taking aregular tube of continuous material and laser etching the tube undercomputer control to form the pattern shown in FIG. 1 a. The stentstructure in FIG. 1 a has a plurality of radially expandable undulatingrings 13-1 through 13-7, each ring containing a plurality of struts,with each directly adjacent pair of radially expandable undulating rings13-i and 13-(i+1) (where “i” is an integer varying from 1 on up) joinedby a plurality of arcuate flex members 12-iA through 12-iF. Thus in FIG.1 a, the first radially expandable undulating ring 13-1 is connected bya plurality of arcuate flex members 12-1A through 12-1F to the secondradially expandable undulating ring 13-2. Two characteristics of theundulating rings are of interest. First each undulating ring is made upof a plurality of U-shaped bent sections with the arm of one U formingan arm of the adjacent oppositely facing U and with consecutive Usopening in opposite directions. The arms of each U are over-bent in thesense that in the sheathed or crimped condition (i.e. the smallestunexpanded condition) the ends of the arms furthest from the end sectionof the U (i.e. the “distal ends”) are closer together than the ends ofthe arms directly attached to the end section of each U (i.e. the“proximal ends”). This over bending assists in allowing the radiallyexpandable undulating ring to expand over a reasonable range withoutsignificantly shortening the stent. The end section of each “U” can becurved, flat, angled, pointed or any other appropriate shape. The tubefrom which the stent shown in FIG. 1 a is formed may be any appropriatediameter and wall thickness. Well known techniques can then be used tocrimp the stent to its desired diameter for insertion into a vessel. Inone embodiment, each radially expandable undulating ring can also beviewed as being made up of nine sinusoidal struts wherein eachsinusoidal strut would start at the middle of an arm of a U and continueuntil it reaches the point where a second sinusoidal strut begins. Thusin FIG. 1 a, a centerline 14 is shown through the center of radiallyexpandable undulating ring 13-1 and the intersection of centerline 14with each arm of each U is identified with the numbers 1 through 18.From FIG. 1 a, it is apparent that the first sinusoid is made up of aportion of the undulating ring extending from number 1 to number 3, thesecond sinusoid is made up of a portion of the undulating ring extendingfrom 3 to 5, the third sinusoid is made up of a portion of theundulating ring extending from 5 to 7 and so on for a total of nine suchsinusoids if the sinusoid extending from 18 to 1 is included when thematerial shown flat has been rolled into a tube.

Stents have a range of unexpanded diameters depending on their intendeduse.

Typically, unexpanded stents range in diameter from fractions of amillimeter on up.

The plurality of arcuate flex members (i.e. “links”) connecting, forexample, undulating ring 13-1 to undulating ring 13-2 comprises six suchmembers as shown in FIG. 1 a.

The arcuate flex section 12-1A extends from an end of arm 1 of the firstsinusoidal strut in ring 13-1 through a sharp S-shape to the adjacentend of arm 3 of an undulating sinusoidal strut member shown inundulating ring 13-2. The arcuate flex member 12-1A (i.e. link A) incircumferential links 12-1 (illustrated by the flex member containedwithin the bracketed section A) is made up of the same material as theradially expandable undulating strut. This section A (as well assections B, C, D, E, and F of links 12-1) expands circumferentially withexpansion of the adjacent rings 13-1 and 13-2 while still holding theradially expandable undulating rings 13-1 and 13-2 in their correctplaces relative to the rest of the stent. The radial/circumferentialexpansion of the stent is accomplished over a substantial distancewithout longitudinally shrinking the stent because of the uniquesinusoidal shape of the radially expandable undulating ring sections 13and the compressed S-shapes of the arcuate flex members 12. Each otherradially expandable undulating ring 13-i is joined to the adjacentradially expandable undulating rings 13-(i−1) and 13-(i+1) by arcuateflex members (12-(i−1) and 12-i (where i is an integer equal to 1 orgreater). For example, circumferential links 12-1A through 12-1F connectrings 13-1 and 13-2. In links 12-1 through 12-6, each arcuate flexmember is as described in connection with arcuate flex members 12-1A or12-1B except that even numbered links 12-2, 12-4, 12-6 et al arecircumferentially offset with respect to odd numbered links 12-1, 12-3,12-5 et al. As a feature of this invention, adjacent pairs of arcuateflex members such as flex members 12-1A and 12-1B are mirrored. Themirroring of the links such as links 12-1A and 12-1B results in a closedunit cell. Any location within a cell has a minimum distance to thematerial defining the boundaries of the cell. The material defining theboundaries of the cell is made up of portions of the links such as 12-1Aand 12-1B as well as portions of the strut materials from two adjacentundulating expansion rings.

FIGS. 1 b, 1 c, 1 d, 1 e, 1 f, 1 g and 1 h show the change in the shapeof each radially expandable undulating ring 13 and the correspondingchanges of the arcuate flex members 12 (i.e. links 12) connectingadjacent radially expandable undulating rings 13 as the stent isexpanded to its full dimension. As can be seen by looking sequentiallyat FIGS. 1 a to 1 h, and focusing, for example, on radially expandableundulating ring 13-1 and arcuate flex members 12-1, the arcuate flexmembers 12-1 extend longitudinally to connect adjacent radiallyexpandable undulating rings such as rings 13-1 and 13-2 thereby to allowthe stent to retain its approximate longitudinal length. The arcuateflex member 12-1A will be identified and labeled in each of FIGS. 1 bthrough 1 h. As can be seen from FIGS. 1 b and 1 c the dimension in thecircumferential direction of circumferentially S′ shaped arcuate flexmember 12-1A increases as the stent expands. This increase causes theflex member 12-1A to form an S with three parallel arms in FIG. 1 b andwith three arms forming small but definitely discernable acute angles θwith each other in FIG. 1 c. FIG. 1 d shows the three arms of the acuteflex member 12-1A forming greater angles θ of approximately 30° witheach other. These angles θ increase substantially as the stent isexpanded circumferentially. In FIG. 1 e the angle θ is approximately 45°and in FIG. 1 f the angle θ is approximately 60°. In FIG. 1 g the angleθ approaches 90°. The arcuate flex member 12-1A in FIG. 1 h has beenexpanded to such an extent that θ is greater than 90°. The arcuate flexmembers represented, for example, by the members 12-1A through 12-1F,give to the stent flexibility during insertion into a lumen,conformability when expanded with and substantially uniform scaffolding.

In one embodiment of this invention the stent material can be a shapememory alloy such as nickel titanium (sometimes called “nitinol”.) Infabricating a stent, the stent is placed in a heat source (at 200° C. to600° C.) such as a salt bath or furnace for from one to thirty minutesto heat the stent material to establish its memory shape, tostress-relieve the stent, and to establish the active A_(f) (i.e. thetemperature at which the material becomes entirely austenite at anappropriate temperature below body temperature such as 25°). Then thelinks are further heated locally to raise A_(f) above the bodytemperature. Such heating can be done using a laser or by applying avoltage to the links for a short time (for example, 0.01 to 1.0 seconds)to raise the A_(f) of the links to above the body temperature. Thiscauses the link material to be predominantly or at least partiallymartensitic at body temperature (about 37° C.) or, for non-stentapplications, at the minimum design temperature. The austenitic shapememory ring material has as its memory state the desired deploymentdiameter. This shape memory alloy is then compressed by, for example,crimping the stent and inserting the stent into a delivery sleeve toform the small unexpanded stent. The stent may be cooled into themartensitic phase before crimping. The stent is then placed in the humanbody. Upon removal of the sleeve when the stent is properly located in alumen or vessel, the stent either has risen or will rise to thetemperature of the body (typically 98.6° F.). This temperature is abovethe temperature at which the ring material changes from martensitic toaustenitic and thus the stent will then expand naturally back to itsoriginal larger circumference thereby avoiding the need for using aballoon catheter.

The portions of the stent to be locally heat treated can be heated in aslittle as one tenth of a second using a laser or by treating each ofthese portions as a resistive wire. Optimum heating for the stent wirewill be less than one second, such as one half second or less. Thisprevents undesireable heating of adjacent portions of the stent. In theaustenitic phase the material assumes a given position. The austeniticmaterial, when chilled, changes to a martensitic phase but retains theshape it had in the austenitic phase. The martensitic material can thenbe crimped, compressed, placed in a sleeve and inserted into a humanvessel. When the sleeve is removed, the material has risen intemperature, becomes austenitic and returns to its normal memoryposition. By appropriately selecting and treating the material to have atransition temperature to the austenitic phase at a suitable intervalbelow the body temperature, a stent can be fabricated which expands uponreaching a body temperature to hold open a vessel or other body part.

Applying additional local heat treatment has the effect of elevating thetransformation temperature A_(f). If the A_(f) of a portion of thedevice is above body temperature, then that portion will be at leastpartially in the martensitic phase. This portion will not besuperelastic at the design temperature and thus will not spring back toits memory state. As a result, this portion can be easily deformed withless force than an identical portion in the austenitic condition. Thisbehavior, when established in a stent link, enhances the flexibility andconformability of the stent. In superelastic stents, there is a commontrade-off between flexibility/conformability and scaffolding/drugdelivery because superelastic links do not deform easily. Byheat-treating the links to be martensitic at body temperature, thistrade-off can be improved; i.e. more links can be used withoutsacrificing flexibility/conformability.

FIG. 2 a shows a cylindrical stent having applied thereto positive andnegative electrical contacts thereby to cause electrical current to flowthrough the portions of the stent material between the positive andnegative contacts. The current flow heats the stent material such thatthe material changes from martensitic to austenitic.

FIG. 2 b shows a positive electrode 21 and a negative electrode 22applied along the longitudinal axis of the stent to cause the stentmaterial between the two contacts to change in phase from martensitic toaustenitic.

FIG. 2 c shows a curved electrode 23 of positive polarity capable ofbeing placed onto a portion of the stent to be converted frommartensitic to austenitic. A corresponding curved electrode 24 ofnegative polarity likewise capable of being placed on the cylindricalstent material to be converted from martensitic to austenitic is shownbelow the stent 20.

FIG. 2 d shows a ceramic cylinder 26 having a conductive wire 25intended to carry a positive voltage running longitudinally along thelength of the ceramic cylinder 26. Conductive wire 25 makes electricalcontact to electrically conductive portions 27-1, 27-2 and 27-3 ofconductive material on the surface of ceramic cylinder 26. Cylinder 26is placed in a stent and then an electrode 28 intended to carry anegative voltage is applied over the portions of the stent material tobe locally heat treated. Electrical current is then passed through thelead 25 to the corresponding positive electrode such as 27-1 and thecurrent then flows through the portions of the stent material to belocally heat treated to negative electrode 28 placed directly adjacentand in contact with these portions of stent material.

Other structures for appropriately applying electrical current to thoseportions of the stent material to be converted from martensitic toaustenitic will be apparent in view of this disclosure.

Also, a laser for this purpose can be used in the manner disclosed in apaper entitled “Laser Annealing of Shape Memory Alloys: A Versatile ToolFor Developing Smart Micro-devices” by Y. Bellouard et al., published inthe Journal of Physics IV France 11 (2001), Pr8-571. See also “LocalAnnealing Of Complex Mechanical Devices: A New Approach For DevelopingMonolithic Micro-devices” by Y. Bellouard et al., published by MaterialsScience And Engineering A273–275 (1999) 795-798.

FIGS. 3 a, 3 b, 3 c and 3 d illustrate cells associated with the stentof this invention (FIG. 3 a) and three prior art stents as well asisomers representing all points having the same minimum distances fromthe cell boundaries.

A parameter of importance in determining the performance of a cell isthe minimum distance from a point within each cell to the closest wallof the cell. A series of isomers, each isomer representing the pointshaving a given minimum distance from a cell wall, can be drawn. Thuseach isomer represents all points having the same minimum distance tothe closest adjacent portion of the strut or link forming the boundariesof the cell. When a series of isomers have been drawn within a cell,there is a point which is further from the cell boundaries than allother points in the cell. This point is called the “least supportedpoint” (hereinafter “LSP”). In the stent of this invention, the distancefrom the LSP to the nearest cell boundary (i.e. to the strut and linkmaterials surrounding the cell) has a smaller such maximum than the LSPin prior art cells. In other words, there is a point on the vessel wallwithin each cell that is least well supported compared to all otherpoints on the vessel wall within the cell. However, as is apparent fromFIG. 1 h, the material making up the stent of this invention isreasonably uniformly distributed over the surface of the vessel whenexpanded. This means that the scaffolding performance of the inventedstent as well as the drug distribution capability of this stent will beenhanced relative to a stent of the prior art.

While some stents are described in terms of “percent metal coverage”(and this is a useful parameter), one problem with this measure is thata stent with a few bulky struts will have the same percent metalcoverage as a stent with many light-weight struts. However, in the areasof vessel wall support and drug delivery, the performance of the twostents will differ significantly. Since various architectures distributethe same metal in dramatically different ways, thereby renderingdifferent results in terms of vessel wall support and drug delivery, theissue is how the metal in the stent is deployed and used. Optimal use ofthe metal affects drug delivery, vessel wall support, insertability ofthe stent into the vessel and the life of the stent.

An optimal stent will restore optimal blood flow through the vessel andprovide radial support while at the same time minimizing vessel injuryduring insertion and expansion and minimizing tissue prolapse. Such astent will maintain patency and optimize delivery of drugs. In order todetermine how successful the stent will be in radially supporting thevessel wall, the size of the holes between the struts and within theboundary of a cell must be determined. Since all cells have differentshapes, the ability of a given cell to support tissue is the issue. Asdescribed briefly above, every cell is such that there is a point (the“LSP”) within the cell farthest away from the closest cell boundary orstrut (i.e. further away from the closest point on the cell boundary).Every other point on the vessel wall is supported by a closer strut thanthis point. In the cells associated with the stents in FIGS. 3 a, 3 b, 3c and 3 d, the distance from the least-supported point in the vesselwall to the nearest strut in the stent of this invention is 0.0168inches. The distances of the least-supported points in the vessel wallto the nearest strut in the cells of FIGS. 3 b, 3 c and 3 d are 0.0236inches, 0.0254 inches and 0.0204 inches, respectively. Thus the stent ofthis invention has a cell which is more efficient in supporting thevessel wall and thus provides better scaffolding and more uniform drugdelivery than these other prior art cells shown in FIGS. 3 b, 3 c, and 3d.

In the cell of this invention shown in FIG. 3 a, the isomers generallycover the whole area of the stent leaving very few places enclosed bythe first isomer to touch itself. In FIG. 3 b, a prior art stent hasisomers but many of the isomers do not extend into a number of theboundary protrusions defining the cell area indicating that this cellwill provide a very high dosage of drugs in these areas if the stent iscoated with a drug for timed release into the vessel.

The cell of FIG. 3 c has the same problem as the cell of FIG. 3 bbecause of the boundary protrusions defining the cell area. Theseprotrusions reflect the very tight coupling of the linkage between thecircumferentially expanded wires associated with the stent.

The cell of FIG. 3 d avoids the protrusions of the cells of FIGS. 3 band 3 c but has large areas of vessel wall less well supported whencompared to the support offered by the cell of this invention. Thus thecell of FIG. 3 d has a number of isomers but the isomer which doublesback and touches itself leaves within its boundaries two large areashaving no continuous isomer extending throughout the whole cell. Thusthis cell has relatively poor scaffolding and druf delivery.

The isomers represent a contour much as in contour lines on atopographical map. When one of these isomers or contour lines offsetfrom the cell parameter touches itself, the area bound by that contouris a measure of how the struts are distributed around the cell area. Thecell of this invention shown in FIG. 3 a, when expanded to 3 mmdiameter, has an area of 0.00093 inch² bound by the contour whichtouches itself. The cell in FIG. 3 b has an area of 0.00427 inch²·boundby the contour that touches itself. The cell in FIG. 3 c has an area of0.00562 inch² bound by the contour which first touches itself and thecell in FIG. 3 d has an area of 0.00267 inch² within the contour whichfirst touches itself. The area within the contour which touches itselfis a measure of how efficiently the stent will support the vessel wallsand of how uniformly any drugs coating the strut material will bedistributed along the vessel wall. As can be seen from this comparison,the cell of this invention has the smallest area surrounded by thecontour which touches itself. This means that this cell provides betterscaffolding and drug delivery than the prior art cells shown.

In order to optimize the antirestenosis therapy, uniform or “even”delivery of drug to the vessel wall is desirable. The point at which acontour or isomer bumps into itself can be considered a point at whichthe tissue in the vessel wall is getting a “double” dose of drugs. Thesmaller the area left inside the contour which first bumps into itselfthe better because this means that there is less area on the vessel wallsupported by the stent which would get a low or inadequate distributionof drug. To the extent this area is left with too low a dosage or drugor without any drug (i.e. left untreated) this area must be minimized.The cell of this invention minimizes this area compared to the cells ofthe prior art of FIGS. 3 b and 3 d. With respect to the cells of FIGS. 3b and 3 c, the cell of this invention minimizes regions of the vesselwall which will receive drug overdoses.

A further measure of the performance of a stent is the ratio of a cell'sperimeter to its internal area. This parameter, which has dimensions of“1/inch” has a value of 109.8 for the cell of one embodiment of thisinvention expanded to a stent diameter of 3 mm. On the other hand, thecells of FIGS. 3 b, 3 c, and 3 d have ratios of cell perimeters tointernal area of 88.4 per inch, 78.5 per inch, and 85.6 per inch,respectively. Therefore the cell of this invention when fully expandedgives more scaffolding per unit area of vessel wall to be supported thando the prior art cells shown in FIGS. 3 b, 3 c, and 3 d.

The parameters of the FIG. 3 a cell of this invention are shown in Table1 along with the corresponding parameters of three prior art cells.

TABLE 1 Distance from Ratio of The area bounded by least-supportedcell's that contour offset point in vessel perimeter to from the cellwall to nearest its internal perimeter which Cell strut (inches) area(in/in²⁾) bumps into itself. FIG. 3a 0.0168 109.8 9.3 FIG. 3b 0.023688.4 42.7 FIG. 3c 0.0254 78.50 56.2 FIG. 3d 0.0204 85.6 26.7

While this invention has been described with reference to illustrativeembodiments and examples, the description is not intended to belimiting. Thus, various modifications of the illustrative embodiments,as well as other embodiments of the invention, will be apparent topersons skilled in the art in view of this description. It is thereforecontemplated that the appended claims will cover any such modificationsor embodiments.

All publications, patents and patent applications referred to herein areincorporated by reference in their entirety to the same extent as ifeach individual publication, patent or patent application wasspecifically and individually indicated to be incorporated by referencein its entirety.

1. An unexpanded stent comprising: a plurality of radially expandableundulating rings, each radially expandable undulating ring comprising astrut with a plurality of apices; a plurality of arcuate flex membersconnecting adjacent first and second radially expandable undulatingrings; the first radially expandable undulating ring comprises a strutwhich is coupled to two circumferentially adjacent arcuate flex memberssuch that more than one complete sinusoidal cycle of strut materialextends between adjacent couplings with said two circumferentiallyadjacent arcuate flex members; and the second radially expandableundulating ring comprises a strut which is coupled to said twocircumferentially adjacent arcuate flex members such that less than onecomplete sinusoidal cycle of the strut material extends between adjacentcouplings with said two circumferentially adjacent arcuate flex members,wherein said arcuate flex members are not connected to a commonsinusoidal strut of either of said first or second undulating rings. 2.The stent of claim 1, wherein each arcuate flex member comprises anS-shaped member.
 3. The stent of claim 2, wherein the S-shaped membercomprises a first apex in substantial longitudinal alignment with anapex of a strut in one of said plurality of radially expandableundulating rings.
 4. The stent of claim 2, wherein the S-shaped membercomprises a second apex in substantial longitudinal alignment with anapex of a strut in a second of said plurality of radially expandableundulating rings.
 5. The stent of claim 2, wherein the S-shaped membercomprises a first apex and a second apex, each of the first apex and thesecond apex in substantial longitudinal alignment with an apex of thefirst and second expandable undulating rings.
 6. The stent of claim 1,wherein adjacent first and second radially expandable undulating ringsare disposed with respect to one another along the stent such thatapices in each of the first and second radially expandable undulatingrings are in longitudinal alignment.
 7. The stent of claim 1, whereinthe flex member connects longitudinally unaligned apices of the strutscomprising adjacent first and second radially expandable undulatingrings.
 8. The stent of claim 1, further comprising a biocompatiblesurface treatment or coating thereon.
 9. The stent of claim 1, furthercomprising a medicinal agent included in or applied to the undulatingrings and the arcuate flex members.
 10. The stent of claim 1, whereinthe plurality of radially expandable undulating rings and the pluralityof arcuate flex members are constructed of a plastically deformablematerial wherein an expansion of said stent to a deployment diameterinvolves substantially plastic deformation.
 11. The stent of claim 1,wherein the plurality of radially expandable undulating rings and theplurality of arcuate flex members are constructed of a material whereinan expansion of said stent to a deployment diameter involvessubstantially elastic deformation.
 12. The stent of claim 1, wherein theplurality of radially expandable undulating rings and the plurality ofarcuate flex members are constructed of stainless steel.
 13. The stentof claim 1, wherein the plurality of radially expandable undulatingstruts and the plurality of arcuate flex members are constructed of ashape memory material.
 14. The stent of claim 1, wherein the pluralityof radially expandable undulating rings and the plurality of arcuateflex members are constructed of nitinol.
 15. A stent of generallycylindrical shape having a longitudinal center axis running the lengthof the stent, comprising in the unexpanded state: a plurality of ringsmade up of a multiplicity of “U” shaped sections, each “U” having an endsection and two arms, with adjacent “U's” in the ring facing in oppositedirections and with each “U” sharing an arm with the two directlyadjacent “U's”; a number of links comprising mirrored pairs connecting aring to an adjacent ring, each link in the mirrored pairs of linkscomprising: a first section connecting to the end section of a selected“U” in the first ring at a point on a first longitudinal line on thesurface of said stent, said first longitudinal line being parallel tosaid axis, a second section connecting to the end section of a selected“U” in the adjacent ring, said connection to the end section of theselected “U” in the adjacent ring being on a second longitudinal line onthe surface of said stent, said second longitudinal line being parallelto said axis and offset from said first longitudinal line, and an “S”shaped section connecting solely to said first section and to saidsecond section; wherein each link in the mirrored pairs of links in amultiplicity of links connecting a pair of rings has its first sectionand second section each connected to an end section of a “U” to which noother link is connected.
 16. The stent of claim 15, wherein the endsection is curved.
 17. The stent of claim 15 wherein the end section isapproximately straight.
 18. The stent as in claim 15, wherein the antisof at least some of the “U's” in the unexpanded stent are such that thedistal ends of the two arms relative to the end section of the “U” arecloser to each other than the proximal ends of the two arms connected tothe end section of the “U”.
 19. The stent as in claim 15 wherein said“S” shaped section is such that the “S” shaped section extendscircumferentially between the two adjacent rings being connected by saidlink.
 20. The stent as in claim 15, wherein each link is capable ofexpanding as the stent expands such that the “S” shaped section becomesgradually straight.
 21. The stent as in claim 20, wherein each link iscapable of expanding such that the angle between the expanded “S” shapedsection and the first section is greater than 90° and the angle betweenthe expanded “S” shaped section and the second section is greater than90°.
 22. The stent of claim 1 wherein the plurality of radiallyexpanding undulating rings and the plurality of arcuate flex memberscomprise a bio-absorbable material.
 23. The stent of claim 22 whereinthe bio-absorbable material is poly-L-lactic acid.
 24. The stent ofclaim 6, wherein said apices in longitudinal alignment point in the samedirection.
 25. The stent as in claim 19 wherein said ‘S’ shaped sectionextends circumferentially such that the connections of said “S” shapedsection to said first and second sections are on different longitudinallines parallel to the longitudinal center axis.